Custom Properties on Children (XAML)

Hi!

I would like to create my own children properties for a custom view, like RelativeLayout does. How this work? Any tip?

<RelativeLayout>
    <BoxView Color="Red"
            WidthRequest="200" HeightRequest="200"
             RelativeLayout.XConstraint= "{ConstraintExpression Type=Constant,  Constant=10}"
             RelativeLayout.YConstraint= "{ConstraintExpression Type=Constant,  Constant=20}" />
</RelativeLayout>

Thanks,
Juli